The present invention relates to a device for determining in the search mode the direction of a low-luminosity emissive source, having a lens forming an image of the emissive source on a photo-sensitive detector constituted by a charge-coupled detection matrix comprising a read register and an output stage of the read register.
A particular advantageous application of the device according to the invention consists in the detection of low-luminosity stars (of great magnitude) in the presence of significant background noise.
With the object of obtaining the two information components (in cartesian coordinates) allowing the complete determination of the observed luminous object, it is known, more specifically from the U.S. Pat. No. 4,430,673 to utilize a photo-sensitive detector constituted by a charge-coupled detection matrix. This detector is in the form of a mosaic of picture elements (pixels) divided into two halves: a first half, the photosensitive zone is intended to receive and detect the luminous radiation coming from the observed object, and the second half, coated or not coated with an opaque film, serves as an analog memory in which, by charge transfer, the information components received by the first half of the matrix during a so-called integration period, are stored. The two halves of the matrix detector preferably have separate charge transfer controls. Thereafter a read register receives, still by charge transfer, line sequentially, the information components contained in the memory zone, thereafter these information components are applied in the analog form to an output stage from which they can be read pixel by pixel in such a manner as to enable the recovery, in digital form, of the image previously obtained on the photosensitive zone. More specifically for celestial observation, a first mode of operation of the device, the search mode, consists in defining, in a detection sub-matrix, what is called the search window incorporated in the matrix described in the foregoing, the position with an accuracy of one pixel, of the desired luminous object, it being understood that the search window was previously defined by means of, for example, an inertial sensor mounted in the same apparatus as the device for sighting the luminous object. Irrespective of its magnitude, the observed luminous object, generally a star, provides, in the detection matrix, a luminous spot whose diameter is less than the diameter of the pixel, it being assumed that the focusing effected by the objective is correct. In the search mode, the pixel which thus receives the total signal from the emissive source (the star) looked for, renders it possible to obtain the above-indicated direction by furnishing the direction of the star by its coordinates with an accuracy of 0.5 time the side of a pixel. However, the search mode is generally followed by a second, so-called measuring mode which has for its object to determine with a still better precision (in a ratio of 10, approximately) the direction of the star. For the measuring mode the star image on the detector matrix is defocused in a such a manner that it covers a surface greater than the surface of a pixel and the energy centre of the star image on the matrix is determined by barycentric calculation of the pixels, more specifically of the pixels subjected to the defocused radiation of the star searched for. In this case, no provisions have been taken to modify the focusing between the search mode and the read mode. The invention can be used in the field in which there is either no defocusing or such a permanent fixed defocusing that the image of the emissive source has a diameter greater than a side of a pixel and preferably being in the order of 2 to 2.5 times this side. The invention is particularly advantageous for use in the event of defocusing.
The precise technical problem encountered in the said field is to collect, in the search window, with an accuracy of the order of one pixel side and with the best possible signal-to-noise ratio, the maximum possible signal of the star searched for.